A transmitter in a radio frequency (RF) communications system modulates an electromagnetic wave carrier signal by impressing information (e.g. voice, image, data, etc.) on a carrier wave having a frequency that can be propagated over the airwaves. In the case of a digital communications system, the information is in the form of a stream of data bits, where each data bit has either a value of “0” or a value of “1”. One commonly used modulation technique is frequency shift keying (FSK). The FSK technique operates by shifting a continuous carrier frequency in a binary manner to either one or the other of two discrete frequencies. One frequency is designated as the “mark” frequency and the other as the “space” frequency. The mark and space frequencies correspond to a binary “1” and a binary “0”, respectively. This FSK modulation scheme (also known as “binary FSK”) is shown in FIG. 1, where the space frequency is indicated by a first frequency shifted carrier of frequency f1, and the mark frequency is indicated by a second frequency shifted carrier frequency of frequency f2.
One particular and popular use of FSK is in radio frequency identification (RFID) systems. Among other applications, RFID systems are used for inventory control, supply chain management, and anti-theft of merchandise in stores. A typical RFID system 20 is shown in FIG. 2. RFID system 20 comprises a plurality of transponders (referred to in the art as “tags”) 200 and one or more transceivers (referred to in the art as “interrogators” or “readers”) 202. A reader 202 includes an antenna 204, which allows it to interrogate one or more of the tags 200 over a forward wireless link. The tags 200 also have their own respective antennas 208, which allow them to transmit tag information back to the reader 202 over reverse wireless link. The reader 202 and other readers (not shown in FIG. 2) communicate with a host computer 210. Data collected from the tags 200 can either be sent directly to the host computer 210 through standard interfaces, or it can be stored in the reader 202 and later uploaded to the host computer 210, either directly or by a wireless link, for data processing.
Tags are typically embodied as semiconductor microchips having small amounts of memory for storing the tag's ID number and, in some applications, information concerning the item to which the tag is associated. Further, tags are either “passive” or “active”, depending on how they are powered. Active tags contain their own on-board power source, i.e. a battery, which the active tag uses to process received signals and to transmit tag information back to a reader. Passive tags do not have batteries. They derive their energy from RF signals broadcast by the reader and electromagnetically coupled to the tag antennae. Part of the coupled electromagnetic energy is rectified and stored in each tag. Passive tags use this stored energy as a power source to operate the logic and the RF modulator so as to send data back to the reader by a technique known as backscatter modulation.
In order for the reader 202 to address any particular tag (i.e. Tag A, B, C, D or E) from the population of tags, a process known as “singulation” is typically used. To singulate a tag from the population of tags, the reader 202 polls the tags 200 for their ID numbers (or derivative thereof), typically on a bit-by-bit basis. Because multiple tag responses may interfere with one another, anti-collision algorithms are typically employed in the singulation process. Anti-collision algorithms are either probabilistic or deterministic. One well-known probabilistic anti-collision algorithm is the Aloha technique, whereby tags respond to a polling signal from the reader 202 at random intervals. If a collision occurs, the tags responsible for the collision wait for another, usually longer, time interval before responding again. A known deterministic anti-collision algorithm is the so-called “binary tree-walking” algorithm. According to this approach, the reader 202 initially polls the tags 200 for the first bit of the tags' respective ID numbers. Based on the bit values received, the reader 202 then limits the number of tags which are to send subsequent bits of their ID numbers. This process is repeated until the ID of a single tag has been singulated.
In an FSK RFID system success of the anti-collision algorithm is conditioned upon the reader being capable of discriminating between the two FSK frequencies employed to represent binary “0's” and binary “1's, both of which may be received at the same time. It would be desirable, therefore, to have a frequency determining apparatus and method capable of determining a dominant frequency contained in simultaneously received signals having multiple frequencies.